Producing silicon steel sheets or strips



- lems with the solution of which Patented May 20, 1941 UNITED. STATES PATE fur-2,234

N'roFHcE raonucme snrcon smear. snna'rs on. s'rmrs Victor W. Carpenter, Franklin, Ohio, assignor to The American Rolling Mill Company, Middletown, Ohio, a corporation of Ohio I No'Drawing. originalapplication January 22, Q

1936, Serial N0. 60,347.

cation October 19, 1 Renewed May 25, 1939 1 Claim. (01. 1489-31) 7 1) Good magnetic'properties,

(2) Satisfactory die life, (3) Absence of coil set, (4) High ductility, and (5) High space factor. The need for good magnetic properties and for a material which, when used as punching or stamping stock, will not wear out the dies too Divided and this appli 935?, Serial No. 106,372.

and while my description will concern itself with strip, I do not desire the appended claim to be 1 limited thereto, excepting where the language exrapidly, and the need for a high space factor, are

clear. Not only does material having no coil set work better in the punch presses, but the presence of coil set is likely to affect magnetic prop- .erties injuriously. Ductility and satisfactory die life are frequently interrelated, but not necessarily so.

I My invention has for its fundamental object the production of silicon steels which are improved in' these respects, and the provision of a process for making them which is highly economical and has a-number of commercial advantages. By low silicon steel is meant a silicon content. generally of not more than 1.5%. Because the market for such products is very highly competitive, it is imperative that any method developed to meet the requirements should be an inexpensive one.

While the requirements for good silicon steel product are clear enough, the difliculties'in attaining such a product are quite complex due to the interplay of the factors involved, as will be pointed out hereinafter. As a consequence, ancillary objects of my invention will be clear to one skilled in the art upon reading these specifications; and the objects of my invention are attained by me in that certain process and product of which I shall now give the aforesaid exemplary embodiment, which has to do with the formation of low silicon steel strip. It should be understood that the teachings herein contained are not limited to the production of strip materials, but are equally applicable to sheets;

' Dressly requires.

For the purpose of comparison with the procedure of my present invention, I now outline an exemplary practice in the art as follows:

(l) Low silicon open hearth heats, containing say .05-.06%. carbon, are hot rolled to .06

The coils are box annealed. The coils are decoiled, roller leveled, recofled and shipped.

Certain disadvantages appear in a process of this kind. The material originally contains say from .05 to .06% carbon, and no effort is made to lower this carbon content. During the final box anneal, large percentages of the carbon mlgrate to the surface and contribute to poor die life. The final box anneal is in itself an expensive and protracted operation. The material is likely to have 0011 set, by which'is meant a permanent curvature after the last annealing, or a curvature in the unstrained portion. If an open anneal is carried on after the box anneal, this adds a step and more expense to th process,

and may require a still further step of pickling.

the strip and the strip is likely to be found worth- Y less commercially.

As distinguished from a process such as this, I follow a process which can be outlined as follows:-

(1) The first step of the process will be the same, viz., the hot rolling of silicon steel to .06

to the second step of the preceding process and constitutesapickle. I

(4) The fourth step of'my process is similar to the third step of the preceding process and comprises cold rolling the material to a gauge approximately 10% greater than the desiredfinished gauge.

(5) The fifth step of myprocess corresponds substantially to the fourth step of the preceding process and comprises an open anneal at a temperature above 1400 F., followed by a pickle.

(6) The sixth step is the cold rolling to final gauge and corresponds to step 5 of the preceding process.

(7) I then substitute anopen anneal at a temperature above 1500' 1'., preferably in a furnace with controlled atmosphere, for the box anneal which constitutes step 7 of the preceding process.

(8) As an eighth step, the material is slit and coiled for shipment.

My new product is a decarburized, cold rolled, open annealed, silicon steel strip which has superior magnetic properties, as measured by A. S. T. M. methods, no coilset, high space factor, high ductility and excellentdle life. On a grade of silicon steel containing forexampie 120 to 1.40% silicon, I have obtained as low a watt loss as .96 watt per pound on 27 gauge strip, and 1.35 watts per pound on 24 gauge strip when tested in the direction of rolling, as cut, at 10 kilogausses, 60 cycles. The straight grain permeability at 16 kilogausses ranged from 500 to 550. In a grade of silicon steel containing, for example, 3.38% silicon, I have obtained as low a watt loss as .68 watt per pound on 29 gauge strip when tested straight grain as cut at 10 kilogausses, 60 cycles, and a straight grain permeability at 16 kilogausses of 800. It will be understood, of course, that core loss increases as the silicon content decreases, and that the figures given above are exemplary only, of materials commercially made in accordance with my process.

In connection with my process, it will be noted at once upon comparison, that it possesses a number of distinct advantages. While it embodies a box annealing step as such, yet this step is carried on at an advantageous point in the cycle as respects commercial economy. It is further to be noted that my box annealing step is also a decarburization step. A number of advantages result from this decarburization, and all three of the prime characteristics necessary in materials of this class are bettered thereby for reasons which will be clear from the foregoing. I have discovered that if silicon steel of approximately the gauge specified is box annealed with the hot mill oxide thereon, the material may very readily be decarburized. Although the hot rolled gauge may range up to .11 inch, I prefer to carry the material down to at least .07 inch, because in the lighter gauges the decarburization may be made complete with a shorter annealing time, and in some instances a continuous box anneal may be used. An excellent gauge for the hot rolled material I have found to be .06 inch. The box annealing temperature may range from 1200 F. to 1700 F.; but I prefer to operate as indicated, between 1350 F. and 1450" F. At these temperatures a relatively short soaking period is necessary, and I have found in general that the soaking period may be shortened to about two hours for low silicon steel, although this may vary with conditions. At 1200 F. however, a 24 hour soak was required for the special atmosphere is required. Even a reducing atmosphere does not appear to prevent the decarburization.

Although I prefer to operate-my box anneal to reduce the carbon to less than .01%, it is not always necessary to carry the decarburization to this extent to meet present specifications for low silicon material.

The material may be pickled in accordance with my third step immediately after the box annealing, or it may be given one cold rolling pass and then pickled, or it may be given a cold rolling pass,' open annealed and then pickled. These alternatives are mentioned for the guidance of the man skilled in the art, nly in caseit is found difilcult to pickle off the reduced scale after the box annealing.

It may be noted at this .point that with the removal of the carbon, and with the cold rolling of the product, which constitutes step 4 of my process, the final annealing for the magnetic properties becomes a rapid open anneal which not only produces a better magnetic material, less expensively, but also produces a material which is not characterized by coil set. I have investigated temperatures for the final open anneal ranging between 1325 degrees I". and 1800 degrees F., and the time must of course be long enough to give good magnetic properties. For example, it requires about five minutes at 1325 for a low silicon material of the type for which specific data have been given above, while at 1700 degrees, the time may be shortened to 30 same material to produce the same eifect. No

seconds. Hence I prefer to anneal at a temperature between 1700 degrees F. and 1800 degrees F., because of the increased production which this renders possible. If the open annealing is carried on in a furnace with controlled atmosphere, no final pickling is necessary, and this still further simplifies and decreases the cost of my process. The formation of a heavy oxide scale on the surface of the strip is likely to be an extraneous cause of poor die life; but if a heavy scale is formed during an open annealing in which the atmosphere was not properly controlled, it is of course, permissible to pickle the strip after the annealing. I

The singular advantage in my process is that if the material is decarburized before the cold rolling, then grain growth and substantially complete strain removal will occur so rapidly in an annealing operation that a short open anneal with its .commercial and metallurgical advantages can replace the customary box anneal. Cementite and pearlite are known to be detrimental to core loss qualities. My new material does not have a significant amount of cementite and pearlite. It also has a relatively very coarse grain.

I have been equally successful in rolling to either 24 gauge or 29 gauge, as well as intermediate gauges. Practically all silicon steel of this grade is rolled to these two gauges.

The fundamental steps in the process of the invention are the steps of decarburizing, cold rolling, and final open annealing. The strain rolling (steps 5 and 6) is optional, it being understood that core loss is improved by the strain rolling treatment. By this treatment is meant the interruption of the cold rolling at a point not far ahead of the desired final gauge for an open anneal (step 5) followed by a cold rolling to final gauge (step 6). By this treatment material more nearly isotropic in magnetic properties'is obtained, having distinct advantages for strain rolling treatment may well be omitted. A

better magnetic material can be-produced from the same stock, however, by including the treatment. Considerations of costmay determine the choice. The best grades from the higher silicon steels should be given the treatment.

The cold rolling step of the strain rolling treatment for'low silicon materials (step 6) may vary, say, from 5 to 12 or 15%; but from 7 to 10% seems to be commercially the most feasible range.

While I have described my process for low silicon materials as defined, it is not so limited. I have used it and determined its effects throughout the range of commercial materials, from silicon contents as low as .15% silicon to as high as 3.40% and 4% silicon.' .As the silicon content varies there are permissible changes in maximum flnal annealing temperatures, as well as the necessary or optimum percentages of strain rolling. It will be within the skill of the worker in the art, following my process to increase the temperatures of annealing as the percentage of silicon increases to obtain the best results, also to reduce the percentage of. the cold rolling in step 6. As the percentage of silicon is increased from .15 to 4.0% the amount of strain rolling may be carried as low as 3%, if desired.

Modifications may be made in my process without departing from the spirit of my invention, as will be clear.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

.Silicon steel strip or sheet stock, having a carbon content not substantially exceeding .0l%,

characterized by the substantial absence of ce-' mentite and pearlite having a coarse grained structure of an average grain size of at least .01 inch, haying freedom from coil set, a core loss which masured in the rolling direction does not vary more than 6% from that measured in any other direction, and further characterized by excellent d-ie life, ductility and space factor, and good magnetic properties substantially approaching isotropic magnetic properties.

VICTOR W. CARPENTER. 

